Ethylene polymers containing pendant metallic carboxylate salt groups have been employed in the preparation of films which are in turn widely used in the packaging field. Films produced from such ethylene polymers are strong and have good barrier properties. A deficiency noted with respect to the use of such films in particular applications is that the films show high water absorption or moisture permeability. Although such water vapor permeability may be accepted for certain applications, this characteristic is particularly undesirable in other fields.
A method for overcoming the moisture permeability of such ethylene polymer films has been described in U.S. Pat. No. 3,511,799. As described therein, aqueous dispersions of self-emulsifiable ethylene polymers containing pendant metallic carboxylate salt groups are contacted with a strongly acidic cation exchange resin charged with ammonium ions and the ammonium ions are exchanged for the metallic cations of the ethylene polymer. The product of the ion exchange process is an ethylene polymer containing pendant ammonium salt groups which readily decompose to the acid. Films produced from these ethylene polymers containing pendant acid groups have low moisture-vapor transmission and excellent heat-seal characteristics and thus have wide application in the packaging field.
Direct preparation of such ethylene polymers containing pendant acid groups, particularly where the concentration of acid groups is relatively high, is extremely difficult. For example, in the preparation of ethylene-acrylic acid copolymers by a polymerization process, the different polymerization rates of the acrylic acid and ethylene monomer moieties and the corrosive action of the acrylic acid makes the commercialization of such direct polymerization processes generally unfeasible. Consequently, ethylene polymers containing high concentrations of pendant acid groups are normally prepared by a two-step process comprising, in the first step, a high-pressure, free-radical polymerization of ethylene and a comonomer containing pendant ester groups. Such high-pressure, free-radical polymerization processes are generally continuous processes conducted in an autoclave or tubular-type reactor. A suitable method for preparing such copolymers is described in U.S. Pat. No. 3,350,372.
In the second step, the ethylene polymers containing pendant ester groups are subjected to a conversion process such as the hydrolysis process described in U.S. Pat. No. 3,485,785. As described therein, ethylene-alkyl acrylate copolymers are hydrolyzed by the action of a metallic base in an aqueous medium at elevated temperatures to provide, in one embodiment, an ethylene polymer containing pendant metallic carboxylate groups. The ethylene polymer containing pendant metallic carboxylate groups is generally self-emulsifiable and dispersed throughout the aqueous medium. Conversion of the pendant carboxylate groups to the acid form is effected by the addition of an acid to the aqueous medium, thereby converting the carboxylate salt groups to the acid form and simultaneously coagulating the dispersed ethylene polymer. To effect conversion of the pendant carboxylate salt groups to the acid form, it has been necessary to change the physical form of the ethylene polymer.
Accordingly, an object of the invention is to provide an improved process for the conversion of pendant carboxylate salt groups of an ethylene polymer to the acid form.
Another object of the invention is to provide a process for the conversion of pendant carboxylate salt groups to the acid form of an ethylene polymer, said ethylene polymer comprising at least the surface of an article of manufacture.
Yet another object of the invention is to provide a process for the conversion of pendant carboxylate salt groups of an ethylene polymer to the acid form wherein said ethylene polymer forms at least the surface of an article of manufacture, said conversion conducted without changing the physical form of the article of manufacture.
Other objects, advantages and features of this invention will be readily apparent to those skilled in the art from the following description and appended claims.